DESCRIPTION (Applicant's Description) Evidence for biological synergism of plasma-membrane tyrosine kinase receptors erbB-3 and erbB-2 in model systems and human breast tumor cell lines raises the possibility that coexpression of these receptors may play a role in tumorigenesis and cell proliferation. Thus, identifying coexpressing erbB-2 and erbB-3 receptors in tumor specimens will provide greater specificity, and might identify a subpopulation of patients with different outcome and chemotherapeutic potential. However, the existing clinical studies are weak, and the data on expression of erbB-3 in breast tumors are inconsistent. In particular, the disagreement among studies with regard to cytoplasmic versus membrane staining is troubling. A major contributing factor is the poorly characterized antibody used in all studies. The goal of this proposal is to validate reagents for detection of erbB-3 expression by immunohistochemistry in paraffin sections of breast tumors. Therefore, our specific aims are: (1) to perform immunostaining of erbB-3 in transfected and tumor cell lines using five different erbB-3 antibodies, (2) to localize the cell fraction in which erbB-3 resides in these cell lines, and (3) to compare the pattern of staining of those antibodies in tumor specimens. Using this knowledge, we can establish a valid protocol for quantitating erbB-3 in breast cancer an evaluating its etiologic prognostic significance.